Heating of braided coronal loops

Abstract

Aims. We investigate the relaxation of braided magnetic loops in order to find out how the type of braiding via footpoint motions affects resultant heating of the loop. Methods. Two magnetic loops, braided in different ways, are used as initial conditions in resistive MHD simulations and their subsequent evolution is studied. Results. The fields both undergo a resistive relaxation in which current sheets form and fragment and the system evolves towards a state of lower energy. In one case this relaxation is very efficient with current sheets filling the volume and homogeneous heating of the loop occurring. In the other case fewer current sheets develop, less magnetic energy is released in the process and a patchy heating of the loop results. The two cases, although very similar in their setup, can be distinguished by the mixing properties of the photospheric driver. The mixing can be measured by the topological entropy of the plasma flow, an observable quantity.

Physical role of topological constraints in localized magnetic relaxation

“…However, our simulations with a net vertical current still have a localized turbulent region (e.g. the α = 0.1 case presented here, or the 'S 3 ' case described by Wilmot-Smith et al [13]). …”

The emergence of braided magnetic fields

“…A second approach to increasing the complexity of the field is to define fields which partly overlap, creating a composite field with more complex internal topology. This is used in what follows to develop a version of the braided field used in Yeates et al (2010), Wilmot-Smith et al (2011), Yeates et al (2015). The technical details of the process by which this field is interpolated onto a Cartesian grid are discussed in Prior and Yeates (2016a).…”

“…Field lines of this composite field are shown in figure 2. It is difficult to see in this curved geometry but sets of the field lines can be shown to form pigtail braids (Wilmot-Smith et al 2011). For this model, the average field strength at t = 0 is B ∼ 20.…”

“…In Wilmot-Smith et al (2011) it was found that the diffuse small scale current structure of this braid leads to rapid and efficient reconnection, causing the field to separate into two twisted flux ropes of opposing chirality. These results were recreated using flux ropes with realistic coronal morphology in Prior and Yeates (2016b).…”

“…For a linear forcefree field, j · B = αB · B and L f is just the linear force-free parameter α, which would be constant throughout the domain. This quantity was used to evaluate the internal structure of relaxing braided and twisted cylindrical fields in Wilmot-Smith et al (2011 and for flux ropes with more realistic coronal geometries in Prior and Yeates (2016a,b). It was found that braided fields, even with significantly reduced Lorentz forces, can often exhibit significantly mixed L f distributions (both positive and negative twisting), indicative of field with complex internal structure.…”